This invention relates to pressure transducers in general and more particularly relates to a pressure transducer apparatus utilizing ceramic substrates or other insulating diaphragms.
The prior art is replete with a number of patents and articles describing various types of pressure transducers for various applications.
Essentially, a widely used type of transducer employs a piezoresistive sensing element or elements which are semiconductor resistors. The piezoresistive effect is well known and semiconductor resistors, when mounted on a suitable diaphragm exhibit a change in resistance in accordance with a change in pressure or force applied to the diaphragm. In regard to such devices the prior art has employed metal and semiconductor diaphragms onto which a piezoresistive element is either mounted or diffused to form suitable transducer structure. Many examples of such structures exist in the prior art and reference is made to U.S. Pat. No. 3,654,579 entitled "ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS" issued on Apr. 4, 1972 to A. D. Kurtz et al and assigned to the assignee herein.
The prior art has also used ceramic materials such as alumina as diaphragms for piezoresistive transducers. According to such techniques, it has been determined that thick film resistors exhibit the piezoresistive effect. In regard to these devices the thick film resistors are deposited on a ceramic diaphragm by means of conventional techniques employed in thick film technology. Based on these techniques, a thick film resistor is screened and fired in a ceramic substrate. These resistors exhibit the piezoresistive effect and in conjunction with the ceramic substrate form transducer structures.
An example of a thick film pressure sensor employing such techniques may be had by referring to U.S. Pat. No. 4,311,980 entitled "DEVICE FOR PRESSURE MEASUREMENT USING A RESISTOR STRAIN GAGE": by M. Prudenziati issued on Jan. 19, 1982.
In any event, based on modern technology, there is a need for a pressure transducer which is extremely reliable in operation and which exhibits a linear operation and small deviations in sensitivity and null offset over a wide range of temperature. It is a further requirement that such transducers have high voltage isolation and further be relatively immune to adverse environmental conditions. Such requirements are needed, for instance, in biomedical transducers such as blood pressure transducers where the transducer must operate in a saline media and be safely employed in the presence of the very high voltages (7,500 v or more) caused by defibrilators. Moreover, leakage currents must be on the order of microamps or less for safety measures. Finally, it is extremely desirable for such a transducer to be disposable and this requirement imposes the need for very low costs of manufacture. This specification addresses a transducer specifically capable because of its novel construction of addressing these and other needs.
A major problem which exists with ceramic transducer as above indicated is the fact that the resultant structures have limited temperature operation. Such limited temperature operation is the result of well known drift. This instability of thick film resistors is due to aging effects, stress relief and plastic flow of the glossy composition of the resistors, electromigration and other effects. Further, such transducers are subjected to breaking or rupturing upon application thereto of large forces and pressures. This is because thick film piezoresitors typically exhibit relatively low gage factors of about 10. Thus, to achieve a satisfactory output operating strain levels are typically high, associated with large applied forces. Semiconductor piezoresistors on the other hand have gage factors of 100 or greater, thus operating strain levels can be reduced.
Diffused silicon piezoresistive transducers are also well known in the prior art. Such transducers also exhibit temperature range due to degradation of P-N junctions employed in their fabrication. Exotic and expensive techniques such as dielectric isolation, silicon or saphire, etc. must be employed to extend their operation. Moreover, such transducers, while very economical to fabricate in small sizes, are very expensive if a large area diaphragm is to be employed. Large area diaphragms are desirable for low pressure applications and also for reducing dependence of output levels on small variations in diaphragm thickness.
Typically, transducers require compensating and normalizing resistors and interconnecting structures. It is an object of the present invention to describe a structure where these elements are economically fabricated integral to the transducer structure by conventional thin and/or thick film techniques.
It is therefore a further objective of the present invention to provide a hybrid pressure transducer which uses a ceramic substrate as a diaphragm and employs silicon piezoresistive devices as the sensing elements. In regard to such structures, one therefore provides an extremely reliable transducer structure which possesses improved stability and linearity over a wide range of temperature and pressure. The resultant hybrid transducer also possesses excellent characteristics in regard to high voltage and high pressure operations and moreover can be manufactured at low cost even if large area diaphragms are required.